Pathophysiology of hepatic encephalopathy.

Changes in various neurotransmitter systems, in blood brain barrier integrity and/or function, and in brain energy metabolism, may precipitate HE. Although a "specific cause" of HE (i.e. the presence of a well-defined toxin) has not been identified so far, the common denominator of all these changes is the presence of liver failure. The various hypotheses of the pathogenesis of HE are not mutually exclusive, but have to be integrated into the clinical syndrome of liver failure. It is conceivable that "specific causes" of HE do not exist at all. Brain function may be affected to a certain degree by the various consequences of liver failure (such as the accumulation of neuroactive compounds and of neurotoxins, changes in the metabolism of amino acids, decreased availability of energy fuels and circulatory changes), finally precipitating the syndrome of HE.     

Neuroactive amino acids in hepatic encephalopathy

There is abundant evidence to suggest that alterations of excitatory and inhibitory amino acids play a significant role in the pathogenesis of hepatic encephalopathy (HE) in both acute and chronic liver diseases. Brain glutamate concentrations are reduced in patients who died in hepatic coma as well as in experimental HE, astrocytic reuptake of glutamate is compromised in liver failure and postsynaptic glutamate receptors (both NMDA and non-NMDA subclasses) are concomitantly reduced in density. Recent studies in experimental acute liver failure suggest reduced capacity of the astrocytic glutamate transporter in this condition. Together, this data suggests that neuron-astrocytic trafficking of glutamate is impaired in HE. Other significant alterations of neuroactive amino acids in HE include a loss of taurine from brain cells to extracellular space, a phenomenon which could relate both to HE and to brain edema in acute liver failure. Increased concentrations of benzodiazepine-like compounds have been reported in human and experimental HE. Clinical trials with the benzodiazepine antagonist flumazenil reveal a beneficial effect in some patients with HE; the mechanism responsible for this effect, however, remains to be determined.     

Albumin dialysis has a favorable effect on amino acid profile in hepatic encephalopathy

According to one popular theory, hepatic encephalopathy (HE) is partly caused by an imbalance in plasma amino acid levels. The Fischer's ratio between branched chain amino acids (BCAAs) and aromatic amino acids (AAAs) correlates with the degree of HE; the lower Fischer's ratio, the higher the grade of HE. Extra-corporeal liver support systems, like MARS(R)-albumin dialysis (Molecular Adsorbents Recirculating System), can improve HE. The MARS(R) system uses a hyperosmolar albumin circuit to remove both water-soluble and albumin-bound substances. Plasma levels of neuroactive amino acids were analyzed in 82 consecutive patients with life-threatening liver failure admitted to our ICU. All patients fulfilled our indications for MARS treatment and most also fulfilled the criteria for liver transplantation (LTx). In patients with acute liver failure (ALF), as compared to those with acute decompensation of chronic liver failure (AcOChr), levels of leucine and isoleucine were significantly higher before MARS(R) treatment. In all patients, before MARS(R) treatment the higher the grade of HE grade the lower was the Fischer's ratio and higher were the levels of inhibitory neuroactive amino acids. During MARS(R) treatments the Fischer's ratio increased, and the grade of HE decreased. The increase in Fischer's ratio was mainly due to the decrease in AAAs. The plasma levels of neuroactive amino acids, methionine, glutamine, glutamate, histidine and taurine decreased during MARS(R)-treatment. In this study MARS(R)-albumin dialysis had a favorable effect on the plasma amino acid profile of patients with HE.     

Neurotransmitter Dysfunction in Hepatic Encephalopathy: New Approaches and New Findings

Hepatic Encephalopathy (HE) is a serious neuropsychiatric condition of both acute and chronic liver failure. Acute liver failure is characterized by rapid evolution of HE and by brain edema. Portal-Systemic encephalopathy (PSE) is particularly prevalent following treatment of portal hypertension or ascites by the TIPS procedure. Available evidence currently suggests that neurotransmission changes rather than brain energy failure are the primary cause of HE. Recent studies both in autopsied brain tissue from HE patients as well as in experimental animal models of HE reveal that liver failure results in altered expression of several genes coding for proteins having key roles in the control of neuronal excitability. Such alterations include decreased expression of the glutamate transporter GLT-1, and increased expression of monoamine oxidase (MAO-A isoform), the peripheral-type benzodiazepine receptor (PTBR) as well as constitutive neuronal nitric oxide synthase (nNOS). Such changes result in altered protein expression and in increased extracellular brain glutamate, increased degradation of monoamine neurotransmitters, increased synthesis of neurosteroids with inhibitory properties, and increased production of nitric oxide (respectively) in brain in chronic liver failure. In the case of GLT-1, PTBR, and nNOS, alterations in expression result from exposure to ammonia and/or manganese, two neurotoxic agents shown previously to be increased in brain in liver failure.     

Raised plasma concentrations of 3-methoxy-4-hydroxyphenylethyleneglycol in cirrhotic patients with or without hepatic encephalopathy.

We measured the plasma concentration of a centrally derived noradrenaline (NA) metabolite, 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), in 20 cirrhotic patients (eight with (group A) and 12 without (group B) hepatic encephalopathy (HE] and in 14 age matched healthy subjects to study if the central NA metabolism would be altered in liver cirrhosis patients, particularly in those with HE. The mean (SEM) plasma MHPG concentrations in the patient groups, group A (74.9 (8.6) pmol/l) and B (54.8 (7.2) pmol/l), were significantly (p less than 0.01) greater than in the control group (22.3 (2.0) pmol/l), and that in group A was significantly (p less than 0.05) greater than in group B. The plasma concentration of MHPG observed in these study subjects (n = 34) correlated (rs = 0.77, p less than 0.01) more strongly with the ratio of plasma catecholamine precursor amino acids (tyrosine and phenylalanine) to other neutral amino acids (tryptophan, leucine, isoleucine, and valine) known to compete with catecholamine precursor amino acids for uptake into the brain than with plasma concentration of tyrosine plus phenylalanine alone (rs = 0.63, p less than 0.01). In addition, the mean plasma MHPG concentrations measured in another group of eight cirrhotic patients (group C) during HE (79.3 (10.6) pmol/l) was significantly (p less than 0.01) greater than that measured after the recovery from HE (47.2 (5.2) pmol/l). The results suggest that the central NA metabolism may be altered in patients with liver cirrhosis, particularly in those with HE, and that the derangement in the central NA metabolism may be associated not only with an increase in plasma catecholamine precursor amino acids but also with a decrease in branched chain amino acids.     

Increased Extracellular Brain Glutamate in Acute Liver Failure: Decreased Uptake or Increased Release?

Glutamatergic dysfunction has been suggested to play an important role in the pathogenesis of hepatic encephalopathy (HE) in acute liver failure (ALF). Increased extracellular brain glutamate concentrations have consistently been described in different experimental animal models of ALF and in patients with increased intracranial pressure due to ALF. High brain ammonia levels remain the leading candidate in the pathogenesis of HE in ALF and studies have demonstrated a correlation between ammonia and increased concentrations of extracellular brain glutamate both clinically and in experimental animal models of ALF. Inhibition of glutamate uptake or increased glutamate release from neurons and/or astrocytes could cause an increase in extracellular glutamate. This review analyses the effect of ammonia on glutamate release from (and uptake into) both neurons and astrocytes and how these pathophysiological mechanisms may be involved in the pathogenesis of HE in ALF.     

Experimental models of hepatic encephalopathy: ISHEN guidelines

Objectives of the International Society for Hepatic Encephalopathy and Nitrogen Metabolism Commission were to identify well‐characterized animal models of hepatic encephalopathy (HE) and to highlight areas of animal modelling of the disorder that are in need of development. Features essential to HE modelling were identified. The best‐characterized animal models of HE in acute liver failure, the so‐called Type A HE, were found to be the hepatic devascularized rat and the rat with thioacetamide‐induced toxic liver injury. In case of chronic liver failure, surgical models in the rat involving end‐to‐side portacaval anastomosis or bile duct ligation were considered to best model minimal/mild (Type B) HE. Unfortunately, at this time, there are no satisfactory animal models of Type C HE resulting from end‐stage alcoholic liver disease or viral hepatitis, the most common aetiologies encountered in patients. The commission highlighted the urgent need for such models and of improved models of HE in chronic liver failure in general as well as a need for models of post‐transplant neuropsychiatric disorders. Studies of HE pathophysiology at the cellular and molecular level continue to benefit from in vitro and or ex vivo models involving brain slices or exposure of cultured cells (principally cultured astrocytes) to toxins such as ammonia, manganese and pro‐inflammatory cytokines. More attention could be paid in the future to in vitro models involving the neurovascular unit, microglia and neuronal co‐cultures in relation to HE pathogenesis.     

Is it time to target gut dysbiosis and immune dysfunction in the therapy of hepatic encephalopathy?

The development of overt hepatic encephalopathy (HE) in a patient with cirrhosis confers a damning prognosis with a 1-year mortality approaching 64%. This complex neuropsychiatric syndrome arises as a consequence of a dysfunctional gut–liver–brain axis. HE has been largely neglected over the past 30 years, with the reliance on therapies aimed at lowering ammonia production or increasing metabolism following the seminal observation that the hepatic urea cycle is the major mammalian ammonia detoxification pathway and is key in the pathogenesis of HE. The relationship with ammonia is more clear-cut in acute liver failure; but in cirrhosis, it has become apparent that inflammation is a key driver and that a disrupted microbiome resulting in gut dysbiosis, bacterial overgrowth and translocation, systemic endotoxemia and immune dysfunction may be more important drivers. Therefore, it is important to re-focus our efforts into developing therapies that modulate the disrupted microbiome or alleviating its downstream consequences.     

Ornithine phenylacetate revisited

In patients with liver failure hyperammonemia is associated with the development of hepatic encephalopathy (HE) and immune impairment. Treatment of hyperammonemia is an unmet clinical need. Ornithine phenylacetate (OP) is a novel drug that is targeted at reducing ammonia concentration in patients with liver disease and therefore a potential treatment for HE. This review describes the mechanism of action of OP and its effect on plasma ammonia levels, brain function and inflammation of OP in both acute and chronic liver failure. Ammonia levels could shown to be reduced for up to 24 h in animal models until 120 h in patients with repeated dosing of the drug. Reduction of plasma ammonia levels is due to the stimulation of ammonia removal in the form of glutamine (through glutamine synthetase), the direct excretion of ammonia in the form phenylacetylglutamine and to a normalisation of glutaminase activity in the gut. Administration of OP is associated with a reduction of brain oedema in rats with chronic bile duct ligation and diminution of intracranial hypertension in a pig model of ALF. Studies to date have indicated that it is safe in humans and trials in overt HE are underway to establish OP as a treatment for this major complication of liver disease.     

A novel plasma filtration therapy for hepatic failure: preclinical studies

There is a need to develop artificial means of liver replacement and/or assistance with the aim of either supporting patients with borderline functional liver cell mass until their liver regenerates, or until a donor liver becomes available for transplantation. Selective plasma filtration is a novel approach to blood purification therapy designed to reduce the level of circulating toxins of hepatic and renal failure, mediators of inflammation and inhibitors of hepatic regeneration. The results of preclinical studies indicate that treatment of pigs with experimentally-induced fulminant hepatic failure is safe and effective in extending survival time and arresting brain swelling. In addition, the amount of ammonia, aromatic amino acids, IL6, TNFalpha and C3a removed during the 6-h treatment in the present study was higher by 34% to 175% than the total plasma content of those substances at the start of therapy.     

Animal Models in the Study of Episodic Hepatic Encephalopathy in Cirrhosis

The availability of an animal model is crucial in studying the pathophysiological mechanisms of disease and to test possible therapies. Now, there are several models for the study of liver diseases, but there still remains a lack of a satisfactory animal model of chronic liver disease with hepatic encephalopathy (HE) and abnormalities in nitrogen metabolism, as seen in humans. In rats, two models of chronic HE are widely used: rats after portacaval anastomosis (PCA) and rats with chronic hyperammonemia. The first one mimics the situation induced in cirrhosis by collateral circulation, and has the problem of the absence of hepatocellular injury. The model of hyperammonemia is useful to study the effect of ammonia as a brain toxic substance, but also lacks liver failure. Bile-duct ligation has been used to induce cirrhosis and could also be a model of HE, probably with the addition of a precipitant factor. An ideal model of HE in chronic liver disease must have liver cirrhosis and a precipitant factor of HE; it must also show neuropathological characteristic findings of HE, neurochemical alterations in the main pathways impaired in these complications of cirrhosis, and low-grade brain edema.     

Brain biomolecules oxidation in portacaval‐shunted rats

Background: Oxidative stress induced by a high ammonia concentration has been suggested to be implicated in the pathophysiology of hepatic encephalopathy (HE). Therefore, oxidative damage of brain biomolecules could contribute towards explaining the neurological and motor alterations observed in HE. Methods: Portacaval‐shunted (PCS) rats (n=5) were used as an animal model of chronic HE. Plasma and brain ammonia were measured by the l ‐glutamate dehydrogenase method. Reactive oxygen species was measured by the dichlorodihydrofluorescein diacetate method. Lipid peroxidation was measured as thiobarbituric acid‐reactive substances (TBARS) by a colorimetric method; malondialdehyde (MDA) and 4‐hydroxy‐2‐noneal (HNE) were measured by HPLC and an immunological method respectively. Protein oxidation (carbonylation) was measured as total carbonyl after labelling with 2,4‐dinitrophenyl hydrazine (DNPH) using a spectrophotometric method. Individual protein oxidation was studied, after labelling with DNPH and its separation by one‐dimensional (1D) electrophoresis, by an immunological method. Results: Ammonia‐induced oxidative stress in PCS rats was associated with increased MDA and HNE, together with increased protein oxidation, evidenced by total carbonyl quantification and by the analysis of individual protein bands separated by 1D electrophoresis. However, lipid peroxidation measured as TBARS did not show differences. Conclusion: Our data show an increased evidence of oxidative stress in PCS rat brain; moreover, PCS rat brain proteins are oxidized (carbonylated), some proteins being more sensitive to oxidation than others. These data also show that at least six specific brain proteins in PCS rats are highly sensitive to carbonylation. Identification of these proteins may be crucial for a better understanding of HE pathophysiology.     

Branched-chain amino acids and muscle ammonia detoxification in cirrhosis

Branched-chain amino acids (BCAA) are used as a therapeutic nutritional supplement in patients with cirrhosis and hepatic encephalopathy (HE). During liver disease, the decreased capacity for urea synthesis and porto-systemic shunting reduce the hepatic clearance of ammonia and skeletal muscle may become the main alternative organ for ammonia detoxification. We here summarize current knowledge of muscle BCAA and ammonia metabolism with a focus on liver cirrhosis and HE. Plasma levels of BCAA are lower and muscle uptake of BCAA seems to be higher in patients with cirrhosis and hyperammonemia. BCAA metabolism may improve muscle net ammonia removal by supplying carbon skeletons for formation of alfa-ketoglutarate that combines with two ammonia molecules to become glutamine. An oral dose of BCAA enhances muscle ammonia metabolism but also transiently increases the arterial ammonia concentration, likely due to extramuscular metabolism of glutamine. We, therefore, speculate that the beneficial effect of long term intake of BCAA on HE demonstrated in clinical studies may be related to an improved muscle mass and nutritional status rather than to an ammonia lowering effect of BCAA themselves.     

Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy

Hepatic encephalopathy (HE) is a common neuro-psychiatric abnormality, which complicates the course of patients with liver disease and results from hepatocellular failure and/or portosystemic shunting. The manifestations of HE are widely variable and involve a spectrum from mild subciinical disturbance to deep coma. Research interest has focused on the role of circulating gut-derived toxins, particularly ammonia, the development of brain swelling and changes in cerebral neurotransmitter systems that lead to global CNS depression and disordered function. Until recently the direct investigation of cerebral function has been difficult in man. However, new magnetic resonance imaging (MRI) techniques provide a non-invasive means of assessment of changes in brain volume (coregistered MRI) and impaired brain function (fMRI), while proton magnetic resonance spectroscopy (1H MRS) detects changes in brain biochemistry, including direct measurement of cerebral osmolytes, such as myoinositol, glutamate and glutamine which govern processes intrinsic to cellular homeostasis, including the accumulation of intracellular water. The concentrations of these intracellular osmolytes alter with hyperammonaemia. MRS-detected metabolite abnormalities correlate with the severity of neuropsychiatric impairment and since MR spectra return towards normal after treatment, the technique may be of use in objective patient monitoring and in assessing the effectiveness of various treatment regimens.     

Brain metabolism of 13N-ammonia during acute hepatic encephalopathy in cirrhosis measured by positron emission tomography

Animal studies and results from 13N-ammonia positron emission tomography (PET) in patients with cirrhosis and minimal hepatic encephalopathy suggest that a disturbed brain ammonia metabolism plays a pivotal role in the pathogenesis of hepatic encephalopathy (HE). We studied brain ammonia kinetics in 8 patients with cirrhosis with an acute episode of clinically overt HE (I-IV), 7 patients with cirrhosis without HE, and 5 healthy subjects, using contemporary dynamic 13N-ammonia PET. Time courses were obtained of 13N-concentrations in cerebral cortex, basal ganglia, and cerebellum (PET-scans) as well as arterial 13N-ammonia, 13N-urea, and 13N-glutamine concentrations (blood samples) after 13N-ammonia injection. Regional 13N-ammonia kinetics was calculated by non-linear fitting of a physiological model of brain ammonia metabolism to the data. Mean permeability-surface area product of 13N-ammonia transfer across blood-brain barrier in cortex, PS(BBB), was 0.21 mL blood/min/mL tissue in patients with HE, 0.31 in patients without HE, and 0.34 in healthy controls; similar differences were seen in basal ganglia and cerebellum. Metabolic trapping of blood 13N-ammonia in the brain showed neither regional, nor patient group differences. Mean net metabolic flux of ammonia from blood into intracellular glutamine in the cortex was 13.4 micromol/min/L tissue in patients with cirrhosis with HE, 7.4 in patients without HE, and 2.6 in healthy controls, significantly correlated to blood ammonia. In conclusion, increased cerebral trapping of ammonia in patients with cirrhosis with acute HE was primarily attributable to increased blood ammonia and to a minor extent to changed ammonia kinetics in the brain.     

Systemic inflammation and ammonia in hepatic encephalopathy

Infection and inflammation have been associated with the development of delirium for many centuries and there is a rapidly growing evidence base supporting the role of inflammation in exacerbating the neurological manifestations of both acute and chronic liver failure. Inflammation in the context of hepatic encephalopathy (HE) can arise directly within the brain itself resulting in astrocytic, microglial and neuronal dysfunction, impacting on the development of ‘brain failure’. Inflammation may also develop systemically and indirectly influence brain function. Systemic inflammation develops following liver injury, resulting in hyperammonemia and a ‘cytotoxic soup’ of pro-inflammatory mediators which are released into the circulation and modulate the impact of ammonia on the brain. The aim of this review is to summarise the current evidence base supporting the synergistic role of systemic inflammation and hyperammonemia in the pathogenesis of hepatic encephalopathy. Systemic inflammation and ammonia induce neutrophil degranulation and release reactive oxygen species into the peripheral circulation that may ultimately cross the blood brain barrier. Circulating endotoxin arising from the gut (bacterial translocation), superimposed sepsis, and hyperammonemia upregulate the expression of microbial pattern recognition receptors such as Toll-like receptors. The early recognition and management of systemic inflammation may not only facilitate improved outcomes in HE but supports the development of novel therapeutic strategies that reduce circulating endotoxemia and immune cell dysfunction.     

Hepatic encephalopathy influences high-affinity uptake of transmitter glutamate and aspartate into the hippocampal formation

The present work was carried out to study the influence of ammonia and factors from sera and cerebrospinal fluid (CSF) from patients with different degrees of chronic liver diseases on [³H]D-aspartate (Asp) and [³H]L-glutamate (Glu) high-affinity uptake into the rat hippocampal formation. For comparison, high-affinity uptake of Glu and Asp was determined in human hippocampal brain tissue obtained at autopsy from cirrhotic patients dying in hepatic coma and from control brains free from neurological, psychiatric, or hepatic diseases. Sera and CSF from patients with chronic liver failure and hepatic encephalopathy (HE) were seen to reduce dramatically Glu and Asp uptake into rat hippocampal dendritic layers. A close inverse relationship was found to exist between the level of ammonia in the sera and the inhibition of uptake, both phenomena correlating highly with the extent of liver failure. The present findings, obtained after dilution of sera from patients with HE while maintaining initial ammonium levels, elucidate, however, that ammonia alone cannot account for the reduction in Glu/Asp uptake capacity. The inhibition of Asp uptake into human hippocampal formation of patients dying in hepatic coma was even more pronounced when compared to that found in rat hippocampus incubated in sera and CSF from patients. Glu/Asp uptake into brain tissue is supposed to be an important factor in the pathogenesis of HE accompanying liver dysfunctions.     

Motor-evoked potentials in awake rats are a valid method of assessing hepatic encephalopathy and of studying its pathogenesis

Experimental models of hepatic encephalopathy (HE) are limited by difficulties in objectively monitoring neuronal function. There are few models that examine a well-defined neuronal pathway and lack the confounding effects of anesthetics. Motor-evoked potentials (MEPs) assess the function of the motor tract, which has been shown to be impaired in patients with cirrhosis. MEPs were elicited by cranial stimulation (central) and compound motor action potential by sciatic nerve stimulation (peripheral) in several models of HE in the rat. The experiments were performed using subcutaneous electrodes without anesthetics. Brain water content was assessed by gravimetry, brain metabolites were measured by magnetic resonance spectroscopy, and amino acids in microdialysates from the frontal cortex were analyzed by high-performance liquid chromatography. Abnormalities of MEP were observed in acute liver failure (ALF) induced by hepatic devascularization in relation to the progression of neurological manifestations. Similar disturbances were seen in rats with portocaval anastomosis after the administration of blood or lipopolysaccharide, but were absent in rats with biliary duct ligation. Hypothermia (≤35°C) and mannitol prevented the development of brain edema in acute liver failure, but only hypothermia avoided the decrease in the amplitude of MEP. Disturbances of MEP caused by the administration of blood into the gastrointestinal tract in rats with portocaval anastomosis were associated with an increase in ammonia, glutamine, and glutamate in brain microdialysate. Conclusion: Assessment of MEP in awake rats is a valid method to monitor HE in models of ALF and precipitated HE. This method shows the lack of efficacy of mannitol, a therapy that decreases brain edema, and relates disturbances of the function of the motor tract to ammonia and its metabolites.     

Hepatic encephalopathy: An approach to its multiple pathophysiological features

Hepatic encephalopathy(HE)is a neuropsychiatric complex syndrome,ranging from subtle behavioral abnormalities to deep coma and death.Hepatic encephalopathy emerges as the major complication of acute or chronic liver failure.Multiplicity of factors are involved in its pathophysiology,such as central and neuromuscular neurotransmission disorder,alterations in sleep patterns and cognition,changes in energy metabolism leading to cell injury,an oxidative/nitrosative state and a neuroinflammatory condition.Moreover,in acute HE,a condition of imminent threat of death is present due to a deleterious astrocyte swelling.In chronic HE,changes in calcium signaling,mitochondrial membrane potential and long term potential expression,N-methyl-D-aspartate-cGMP and peripheral benzodiazepine receptors alterations,and changes in the mRNA and protein expression and redistribution in the cerebral blood flow can be observed.The main molecule indicated as responsible for all these changes in HE is ammonia.There is no doubt that ammonia,a neurotoxic molecule,triggers or at least facilitates most of these changes.Ammonia plasma levels are increased two-to three-fold in patients with mild to moderate cirrhotic HE and up to ten-fold in patients with acute liver failure. Hepatic and inter-organ trafficking of ammonia and its metabolite,glutamine(GLN),lead to hyperammonemic conditions.Removal of hepatic ammonia is a differentiated work that includes the hepatocyte,through the urea cycle,converting ammonia into GLN via glutamine synthetase.Under pathological conditions,such as liver damage or liver blood bypass,the ammonia plasma level starts to rise and the risk of HE developing is high. Knowledge of the pathophysiology of HE is rapidly expanding and identification of focally localized triggers has led the development of new possibilities for HE to be considered.This editorial will focus on issues where, to the best of our knowledge,more research is needed in order to clarify,at least partially,controversial topics.     

Pathophysiology of Hepatic Encephalopathy: A New Look at Ammonia

Results of neuropathologic, spectroscopic, and neurochemical studies continue to confirm a major role for ammonia in the pathogenesis of the central nervous system complications of both acute and chronic liver failure. Damage to astrocytes characterized by cell swelling (acute liver failure) or Alzheimer Type II astrocytosis (chronic liver failure) can be readily reproduced by acute or chronic exposure of these cells in vitro to pathophysiologically relevant concentrations of ammonia. Furthermore, exposure of the brain or cultured astrocytes to ammonia results in similar alterations in expression of genes coding for key astrocytic proteins. Such proteins include the structural glial fibrillary acidic protein, glutamate transporters, and peripheral-type (mitochondrial) benzodiazepine receptors. Brain–blood ammonia concentration ratios (normally of the order of 2) are increased up to fourfold in liver failure and arterial blood ammonia concentrations are good predictors of cerebral herniation in patients with acute liver failure. Studies using ¹H magnetic resonance spectroscopy in patients with chronic liver failure reveal a positive correlation between the severity of neuropsychiatric symptoms and brain concentrations of the brain ammonia-detoxification product glutamine. Increased intracellular glutamine may be a contributory cause of brain edema in hyperammonemia. Positron emission tomography studies using ¹³HN₃ provide evidence of increased blood–brain ammonia transfer and brain ammonia utilization rates in patients with chronic liver failure. In addition to the use of nonabsorbable disaccharides and antibiotics to reduce gut ammonia production, new approaches to the treatment of hepatic encephalopathy by lowering of brain ammonia include the use of L-ornithine–L-aspartate and mild hypothermia.